Method for measuring a substrate for semiconductor lithography

ABSTRACT

A method for measuring a substrate for semiconductor lithography using a measuring device, wherein the measuring device comprises a recording device for capturing at least a partial region of the substrate and, wherein the distance between the substrate and an imaging optical unit of the recording device is varied while the partial region is captured by the recording device.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present patent application claims priority from German patentapplication DE 10 2021 128 222.5, filed on Oct. 29, 2022, the content ofwhich is fully incorporated by reference herein.

TECHNICAL FIELD

The invention relates to a method for measuring a substrate forsemiconductor lithography.

BACKGROUND

In the metrology of objects such as substrates, which may be formed forexample as photomasks for semiconductor lithography, defocusinginformation, that is to say how imaging changes when there are differingdegrees of defocusing of the imaging, is of great importance in themeasurement of the substrates. Usually, a so-called focus stack of theimage representation of the substrate is produced by the moving of themeasuring table or the support on which the substrate lies, that is tosay a number of image representations are recorded one after the otherfor different z positions of the measuring table or the support; the zdirection in this case corresponds substantially to the normal on thefocal plane of the optical system used for measurement purposes. Theindividual image representations of the focus stack can then be used forexample to make an assertion about the so-called printability, that isto say the correct imaging of an object onto a wafer in a projectionexposure apparatus over a region around the best focus. The best focusis in this case the distance of the object from the imaging optical unitat which the image is in sharpest focus.

The determination of the position of one or more partial regions of theobject, the so-called regions of interest (ROI), is a furtherapplication in which the focus stack is used. Usually, a focus stack iscaptured for each partial region in order to determine the best focus ofthe imaging device and this focus stack is used as a basis fordetermining the precise position of the partial region. If a pluralityof partial regions are situated within the captured region of thedetector unit, the so-called field of view (FoV), then the individuallydifferent positions of the individual partial regions may also bedetermined from a single focus stack.

The image representations of the focus stack should be captured asquickly as possible in order to ensure a high throughput. One way ofproducing the focus stack known in the prior art, in which a regionaround the expected focus in the imaging direction (z-direction), whichis also referred to as the defocus region, is measured by way of manyindividual images at defined positions and used to determine the bestfocus and the precise position of the partial region on the object, doesnot, however, satisfy the conditions in respect of throughput.

SUMMARY

In general, in one aspect, disclosed is a method for measuring asubstrate for semiconductor lithography using a measuring device, themeasuring device comprising a recording device for capturing at least apartial region of the substrate, and in the method the distance betweenthe substrate and an imaging optical unit of the recording device isvaried while the partial region is captured by the recording device.This is advantageous in that there is no need to take times forpositioning procedures, that is to say the displacement from a firstpredetermined measurement position to a further predeterminedmeasurement position and the settling time for calming the system oncethe further measurement position has been reached, into considerationbetween the capture of the at least two image representations. Theunsharpness of the image representation produced as a result of varyingthe distance during the capture of the image representation can becompensated by the image evaluation tools used in mask inspection ofmicroscopes.

Furthermore, the distance can be varied at a constant speed. This isadvantageous in that each captured image representation, which has anexposure time of 200 ms for example, always includes the same change indistance.

In particular, the distance between the substrate and the imagingoptical unit can be varied by way of a movable object stage which holdsthe substrate. The latter can be positioned in lateral and verticaldirection with an accuracy of 100 nm to 20 nm such that, initially, apartial region of the substrate to be measured is brought into thecaptured region of the recording device and subsequently a positionbelow or above the first measured position of a so-called focus stack,that is to say a plurality of image representations about a best focus,is homed in on. The movement of the object stage is preferably from thebottom upward, that is to say against the direction of gravity, as thisadvantageously allows play present in mechanical drives or a hysteresisin the drive to be minimized.

In the process, the position of the object stage can be detectedcontinually while the object stage is moved. As a result, it ispossible, in a targeted manner, to evaluate only the vertical andlateral positions corresponding to the time interval in which the imagerepresentations are captured, as a result of which the accuracy of theevaluation is advantageously increased.

In a first embodiment, the partial region of the substrate can becaptured by the recording device by way of capturing individual imagerepresentations. Within the meaning of the invention, this should beunderstood to mean that each capture of an image representation isactively started by a pulse from outside of the recording device, forexample from a controller of the measuring device, in a mannercomparable to triggering a photography camera. Technology-induced idletimes may occur between the capture of the image representations, forexample for reading the buffer arranged on a CCD chip of the recordingdevice.

In a further embodiment, the image representations can be capturedcontinually by the recording device. Within the meaning of theapplication, continually is understood to mean that all captured imagerepresentations are captured in succession without a new pulse fromoutside of the recording device, for example from the controller of themeasuring device. If necessary, the capture of the individual imagerepresentations is only interrupted for a period of time, for example200 ns, necessitated by technology, during which the captured signalsare moved into a buffer formed on the CCD chip of the recording device.This type of capture is also referred to as video mode.

In particular, all image representations captured by the recordingdevice are used to determine the position of the partial region on thesubstrate. This advantageously minimizes the overall time required todetermine the position of the partial region on the substrate.

Furthermore, the recording device may be started by a trigger fromoutside the recording device. In this case, the overall controller ofthe measuring device or the control of the object stage may trigger thetrigger. In this case, the trigger should be understood to mean a startsignal for starting a process step, in this case the start of therecording device.

In particular, the trigger can be triggered after a constant speed ofthe object stage and a first predetermined measurement position arereached. This is advantageous in that capturing of the imagerepresentations is only started once the preconditions for using theimage representation are met, which in this case are a constant speed ofthe object stage and, resulting therefrom, a variation in the distancebetween the substrate and the optical unit of the recording device, andthe arrival at the first measurement position of the focus stack.

Furthermore, an illumination of the partial region can be started on thebasis of a signal level output by the recording device and the positionsof the object stage detected during the illumination can be marked. Theillumination, which comprises a pulsed laser for example, is activatedif a certain threshold value of the signal level is exceeded.

In particular, only the image representations recorded at the markedpositions of the object stage might be used for the evaluation todetermine the position of the partial region on the substrate. Theassignment of the positions to the image representations which can beused to evaluate the individual image representations advantageouslyincreases the accuracy and reproducibility of the determination of theposition of the partial region on the substrate.

Furthermore, the image representations that are out-of-focus as a resultof varying the distance between the substrate and the optical unit ofthe recording device can be evaluated on the basis of a statisticalmethod. In the case of mask inspection microscopes as already known fromthe prior art, the accuracy and reproducibility required for determiningthe position of the partial region on the substrate is usually a factorof 10 to 50 below the resolution of the utilized recording devices, suchas a CCD camera for example. As a result, the image evaluation alreadyapplies methods and algorithms which evaluate out-of-focus imagerepresentations within the meaning of the application. The additionalunsharpness as a result of varying the distance has no influence on theaccuracy or reproducibility of the determined positions of the partialregion on the substrate since the positional variations can bedetermined during the image recording and can be taken into accountwithin the scope of the image processing. As a result of the whencapturing conventionally seven image representations for determining thebest focus, it is possible to achieve the same accuracy andreproducibility within a shorter period of time. Alternatively,additional image representations may be captured, for example a total offifteen image representations, if the same amount of time is used as isrequired in the prior art for the capture of individual imagerepresentations. As a result, the accuracy and reproducibility canadvantageously be increased by up to 25% on the basis of the algorithmsbased on statistics that are used during the evaluation.

All documents referred to herein, if any, are incorporated by referencein their entirety. In case of conflict with the present disclosure, andany document incorporated by reference, the present disclosure controls.

The details of one or more embodiments of the are set forth in theaccompanying drawings and the description below. Other features,objects, and advantages will be apparent from the description anddrawings, and from the claims.

DESCRIPTION OF DRAWINGS

Exemplary embodiments and variants are explained in more detail belowwith reference to the drawing, in which

FIG. 1 shows a schematic structure of a device from the prior art,

FIGS. 2A and 2B show a timing diagram known from the prior art and atiming diagram according to the invention, in each case for an imagecapture, and

FIGS. 3A and 3B show a flowchart known from the prior art and aflowchart according to the invention, in each case for an image capture.

DETAILED DESCRIPTION

FIG. 1 shows a schematic representation of a mask inspection microscope1 known from the prior art, which is used for measuring a substrate inthe form of a semiconductor lithography structure 7, which may be in theform of photomask for example, and in which the invention may beimplemented. The mask inspection microscope 1 comprises two lightsources 3, 4, with a first light source 3 being designed for ameasurement of the semiconductor lithography structure 7 in reflectionand a second light source 4 being designed for a measurement of thesemiconductor lithography structure 7 in transmission. The semiconductorlithography structure 7 is arranged on an object stage 6, which canposition the semiconductor lithography structure 7 laterally andvertically. In this case, the positional accuracy can be in particularin a range of less than 100 nm, in particular less than 20 nm. During atransmitted-light measurement, the measurement light 13 of theillumination unit 14, which comprises the light source 4 and anillumination optical element embodied as a condenser 5, passes throughthe condenser 5, which generates a desired light distribution on thesemiconductor lithography structure 7. The measurement light 13 passesfurther through the semiconductor lithography structure 7, which issubsequently imaged by an imaging optical element 8 and a tube 10. Thetube 10 magnifies the image representation of the semiconductorlithography structure 7 and images it in turn on a recording device 2embodied as a CCD camera, which is used to capture the imagerepresentations. The partly transparent mirror 9 arranged between theimaging optical element 8 and the tube 10 is used for the measurement inreflection and has no influence on the measurement in transmitted light.

During a measurement in reflection, the measurement light 12 emitted bythe light source 3 is reflected at the partly transparent mirror 9 andthen impinges on the imaging optical element 8. The latter focuses themeasurement light 12 on the semiconductor lithography structure 7, fromwhich it is reflected. The measurement light 12 passes once more throughthe imaging optical element 8 and the latter images the semiconductorlithography structure 7 through the semitransparent mirror 9 and thetube 10. The tube 10 magnifies the image representation of thesemiconductor lithography structure 7 and images it on the recordingdevice 2. The mask inspection microscope 1 comprises a controller 11,which controls and/or regulates the positioning of the object stage 6and the switchover between a measurement in reflection and atransmitted-light measurement, and which is also used to evaluate theimage representations captured by means of the mask inspectionmicroscope 1.

FIG. 2A shows a timing diagram of a displacement, known from the priorart, of at least two image representations for determining a best focusor for measuring the position of a partial region on the semiconductorlithography structure 7. Usually, 7 to 15 image representations, eachwith a different distance between the object and the optical unit of amask inspection microscope 1 as described in FIG. 1 , are recorded todetermine the best focus, which is why these image representations arealso referred to as a focus stack. To create such a focus stack, theobject stage 6 depicted in FIG. 1 is positioned at a predeterminedposition by the controller 11, with said object stage being moved in thelateral and vertical direction to this end. In this case, lateral issubstantially parallel to the focal plane of the mask inspectionmicroscope 1, with the perpendicular direction also being referred to asz-direction. The illumination of the partial region which is representedby the lower most line, denoted by A, in the timing diagram of FIG. 2Ais started. At the same time, a CCD chip of the recording device 2designed as a CCD camera captures the image representation imaged by theoptical unit of the mask inspection microscope 1 onto a CCD chip of therecording device 2. The illumination time is 200 ms in the exemplaryembodiment shown. After the 200 ms have expired, the illumination, whichcomprises a laser for example, is deactivated and the signals capturedby the CCD chip are shifted to the buffer present directly on the CCDchip during a time interval of 200 ns, as depicted in the timing diagramin FIG. 2A in the line denoted by B. From there, the signals are read in150 ms, as depicted in the timing diagram in FIG. 2A in the line denotedby C. Subsequently, in a further 80 ms, the signals are transferred tothe controller 11 of the mask inspection microscope 1, already explainedin relation to FIG. 1 , in the line denoted by D by way of a connection,for example by way of what is known as FireWire, USB, GPIB or an opticaldata transfer. The image representations are evaluated and, after thelast image representation has been captured and evaluated, the positionof the partial region on the semiconductor lithography structure 7 isdetermined from all image representations of the focus stack in saidcontroller. The second image representation can only be captured oncethe signals from the capture of the first image representation have beenread from the buffer, that is to say after the step depicted in line Cof the timing diagram. By contrast, transmitting the signals to thecontroller 11 (line D) can be carried out in parallel with the captureof the second image representation. In the time interval following thefirst capture of the image representation, the object stage 6 moves inthe vertical direction of the optical axis of the mask inspectionmicroscope 1 to the subsequent position of the focus stack. The latteris usually reached within the 150 ms required to read the CCD chip,which time includes a settling time required after the position has beenreached, that is to say the time until the system is stationary againfollowing the pulse brought about by the displacement.

FIG. 2B shows a timing diagram of a method according to the invention,which depicts the temporal sequence of the individual method steps. Incontrast to the individual capture of the image representations asexplained in relation to FIG. 2A, the image representations are capturedin a video mode represented in line A′ in the timing diagram. In thiscontext, video mode means that the capture of the image representations,once started, is implemented continually. The capture of the imagerepresentations is only interrupted by a transfer of the capturedsignals into the buffer situated on the CCD chip, as depicted in line Bin the timing diagram and as takes 200 ns in this example. Theillumination duration still is 200 ms, with the latter and the number ofimage representations recorded in the video mode being able to be set asa matter of principle. In this mode, the next image recording startssimultaneously with the readout of the signals from the buffer (line C).The transfer times (line D) of the signals from the CCD chip to thecontroller 11 are parallel in time with the capture of the imagerepresentations. An optimal image recording rate is obtained if theillumination time is longer than or equal to the readout time of thesignals. Moreover, the object stage 6 is moved continuously, inparticular at a constant speed, in the vertical direction, as a resultof which the image representations are recorded while the object stage 6is in motion. The image representations which are out of focus as aresult are corrected during the evaluation of the image representationsin the controller 11. The accuracy and reproducibility required fordetermining the position of the partial region on the semiconductorlithography structure 7 is usually a factor of 10 to 50 below theresolution of the recording devices 2 used in mask inspectionmicroscopes 1, and so the image evaluation already evaluates imagerepresentations that are out of focus within the meaning of theapplication, even in the case of the method known from the prior art.The method according to the invention is advantageous in that the timerequired for positioning and for the settling time are reduced towardzero as a result of the continual motion of the object stage (even inthe case of single image recordings). The additional use of the videomode is furthermore advantageous in that the readout time still requiredin the case of an individual capture of the image representations isreduced to the minimum required from a technological point of view.

FIG. 3A shows a flowchart of the method known from the prior art, whichwas explained in FIG. 2A on the basis of a timing diagram. The objectstage 6 is displaced to a predetermined lateral and vertical position ina first method step. After the position has been reached within apredetermined tolerance, the control of the object stage 6 provides atrigger signal in the form of a pulse to the illumination control and tothe control of the recording device 2, which are both partial systems ofthe controller 11 explained in FIG. 1 , in a second method step. As aresult, the illumination, for example a laser, and the capture of theimage representation are started for a fixed duration, which is 200 msin the embodiment explained in FIG. 2A, in a third method step. Afterthe image representation has been captured, the signals required for theevaluation, for example the position of the object stage 6, and thesignals of the captured image representation are transmitted to thecontroller 11 in a fourth method step. The controller 11 evaluates thesignals in a fifth method step. At the same time, the controller 11provides a signal to the control of the object stage 6 and the objectstage 6 homes in on the next predetermined position, where theabove-described method repeats. This is carried out until the lastposition, denoted by N in FIG. 3A, is reached. Once all imagerepresentations have been captured, transferred and evaluated, theposition of the partial region on the substrate is determined on thebasis of all captured image representations, with algorithms based onstatistical methods being used to this end.

FIG. 3B shows a flowchart of the method according to the invention,which was explained in FIG. 2B on the basis of a timing diagram. Incontrast to the flowchart explained in FIG. 3A, the object stage 6, in afirst method step, initially homes in on a position located just aboveor below the region to be captured, with the positions for capturing theimage representations of the focus stack for the purposes of determiningthe best focus preferably being passed through from bottom to top, thatis to say against gravity. Proceeding from this starting position, thecontinual movement of the object stage 6 is started in a second methodstep, with the illumination and the recording device 2 having a statusthat allows the illumination or the recording device 2 to be startedwithout time delay. As soon as the object stage 6 moves at a constantspeed and the vertical position for the first image representation ofthe focus stack has been reached, a trigger in the form of a pulse istransmitted to the recording device 2 by the controller of the objectstage 6 in a third method step. The said recording device 2 in turnoutputs a signal level, on the basis of which - that is to say when acertain threshold value is exceeded - the start of the capture in thevideo mode is initiated in a fourth method step. Furthermore, theillumination is activated when the threshold value is exceeded, and theconstant lateral and continually changing vertical z-position iscaptured by the control of the object stage 6. The signal level of therecording device 2 is above the threshold value for as long as therecording device 2 is in the process of capturing the imagerepresentation, and is below the threshold value for 200 ns when therecording device 2 writes the signals captured by the recording device 2into the buffer or when the predetermined number of imagerepresentations have been captured following the illumination time, thatis to say after 200 ms according to the method explained in FIG. 2B.

Expressed differently, the threshold value being overshot causes theillumination to be started and the continually captured position data tobe marked during a capture of an image representation. The thresholdvalue being undershot causes the illumination to be stopped, that is tosay no light falls on the substrate 7, and causes the controller todetect that the position data captured thereby are captured withoutillumination. In a fifth method step, the illumination is deactivatedduring the readout into the buffer since a continual exposure of the CCDchip used in the recording device 2 may lead to falsification of themeasurement results as it is not possible in this case to ensure thatthe same amount of light is captured for each image representation. Thisis related to the above-described functionality of the buffer on the CCDchip, which is read line-by-line, as a result of which the light isdistributed among two images and leads to an unwanted uneven brightnessdistribution. As described further above, the control of the objectstage 6 acquires all position data, with only the position datacorresponding to the captured image representations being used for thesubsequent evaluation of the individual image representations. Asdescribed, positions captured during the readout into the buffer of theCCD chip can be retrospectively identified and can be left unconsideredduring the calculation of the positions. The signal level is againreturned to high - that is to say above the threshold value - with thestart of the next image representation, as a result of which theillumination is started and the control of the object stage 6 acquiresthe position data during the imaging. Once the last image representationis complete, the object stage 6 is stopped and the evaluation of thefocus stack starts. As a result of its faster succession during thecapture of the image representations, the method according to theinvention is advantageous in that either the same number of imagerepresentations can be captured within a shorter period of time or ahigher accuracy and reproducibility can be achieved during the sameamount of time. The determination of the position of the partial regionfrom an out-of-focus image representation is based, inter alia, onstatistical considerations, as a result of which more data leads to animproved or more accurate determination of the position.

While the disclosure has been described in connection with certainembodiments, it is to be understood that the disclosure is not to belimited to the disclosed embodiments but, on the contrary, is intendedto cover various modifications and equivalent arrangements includedwithin the scope of the appended claims, which scope is to be accordedthe broadest interpretation so as to encompass all such modificationsand equivalent structures as is permitted under the law.

List of reference signs 1 Mask inspection microscope 2 Recording device,CCD camera 3 Light source for reflection 4 Light source for transmittedlight 5 Condenser 6 Object stage 7 Semiconductor lithography structure,especially photomask or wafer 8 Imaging optical unit 9 Mirror 10 Tube 11Controller 12 Measurement light (reflection beam path) 13 Measurementlight (transmitted-light beam path) 14 Illumination unit A Illuminationtime A′ Illumination time B Storage on the detector C Readout time ofthe detector D Transfer time to the controller

What is claimed is:
 1. A method for measuring a substrate forsemiconductor lithography using a measuring device, the measuring devicecomprising a recording device for capturing at least a partial region ofthe substrate, wherein the distance between the substrate and an imagingoptical unit of the recording device is varied while the partial regionis captured by the recording device.
 2. The method of claim 1, whereinthe distance is varied at a constant speed.
 3. The method of claim 1,wherein the distance between the substrate and the imaging optical unitis varied by way of a movable object stage which holds the substrate. 4.The method of claim 3, wherein the position of the object stage isdetected continually while the object stage is moved.
 5. The method ofclaim 1, wherein the partial region of the substrate is captured by therecording device by way of capturing individual image representations.6. The method of claim 1, wherein the image representations are capturedcontinually by the recording device.
 7. The method of claim 5, whereinall captured image representations are used to determine the position ofthe partial region of the substrate.
 8. The method of claim 1, whereinthe recording device is started by a trigger from outside the recordingdevice .
 9. The method of claim 8, wherein the trigger is triggeredafter a constant speed of the object stage and a first predeterminedmeasurement position are reached.
 10. The method of claim 9, wherein anillumination of the partial region is started on the basis of a signallevel output by the recording device and the positions of the objectstage detected during the illumination are marked.
 11. The method ofclaim 10, wherein only the image representations recorded at the markedpositions of the object stage are used for the evaluation to determinethe position of the partial region on the substrate.
 12. The method ofclaim 1, wherein the image representations that are out of focus as aresult of varying the distance are evaluated on the basis of astatistical method.
 13. The method of claim 6, wherein all capturedimage representations are used to determine the position of the partialregion of the substrate.
 14. The method of claim 4, wherein therecording device is started by a trigger from outside the recordingdevice.
 15. The method of claim 14, wherein the trigger is triggeredafter a constant speed of the object stage and a first predeterminedmeasurement position are reached.
 16. The method of claim 15, wherein anillumination of the partial region is started on the basis of a signallevel output by the recording device and the positions of the objectstage detected during the illumination are marked.
 17. The method ofclaim 16, wherein only the image representations recorded at the markedpositions of the object stage are used for the evaluation to determinethe position of the partial region on the substrate.
 18. The method ofclaim 12, wherein the distance between the substrate and the imagingoptical unit is varied by way of a movable object stage which holds thesubstrate.
 19. The method of claim 18, wherein the position of theobject stage is detected continually while the object stage is moved.20. The method of claim 12, wherein the recording device is started by atrigger from outside the recording device.